This invention is generally in the field of Nuclear Magnetic Resonance (NMR) based techniques, and relates to a device and method for magnetic resonance imaging (MRI). Although not limited thereto, the invention is particularly useful for medical purposes, to acquire images of cavities in a human body, but may also be used in any industrial application.
MRI is a known imaging technique, used especially in cases where soft tissues are to be differentiated. Alternative techniques, such as ultrasound or X-ray based techniques, which mostly utilize spatial variations in material density, have inherently limited capabilities in differentiating soft tissues.
NMR is a term used to describe the physical phenomenon in which nuclei, when placed in a static magnetic field, respond to a superimposed alternating (RF) magnetic field. It is known that when the RF magnetic field has a component perpendicular in direction to the static magnetic field, and when this component oscillates at a frequency known as the resonance frequency of the nuclei, then the nuclei can be excited by the RF magnetic field. This excitation is manifested in the temporal behavior of nuclear magnetization following the excitation phase, which in turn can be detected by a reception coil and termed the NMR signal. A key element in the utilization of NMR for imaging purposes is that the resonance frequency, known as the Larmor frequency, has a linear dependence on the intensity of the static magnetic field in which the nuclei reside. By applying a static magnetic field, of which the intensity is spatially dependent, it is possible to differentiate signals received from nuclei residing in different magnetic field intensities, and therefore in different spatial locations. The techniques, which utilize NMR phenomena for obtaining spatial distribution images of nuclei and nuclear characteristics, are termed MRI.
In conventional MRI techniques spatial resolution is achieved by superimposing a stationary magnetic field gradient on a static homogeneous magnetic field. By using a series of excitations and signal receptions under various gradient orientations a complete image of nuclear distribution can be obtained. Furthermore, it is a unique quality of MRI that the spatial distribution of chemical and physical characteristics of materials, such as biological tissue, can be enhanced and contrasted in many different manners by varying the excitation scheme, known as the MRI sequence, and by using an appropriate processing method.
The commercial MRI-based systems suffer from the relatively low signal sensitivity that requires long image acquisition time. Moreover, these systems are expensive and complicated in operation. These drawbacks become more essential when an MRI system is used for imaging relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field over the entire imaged volume, leading to extensive equipment size. Additionally, the unavoidable distance between a signal receiving coil and most of the imaging volume significantly reduces imaging sensitivity.
There are a number of applications in which there is a need for imaging of relatively small volumes, where some of the above-noted shortcomings may be overcome. One such application is geophysical well logging, where the xe2x80x9cwhole bodyxe2x80x9d MRI approach is obviously impossible. Here, a hole is drilled in the earth""s crust and measuring equipment is inserted thereinto for local imaging of the surrounding medium at different depths.
Several methods and apparatuses have been developed aimed at extracting NMR data from the bore hole walls, including, U.S. Pat. Nos. 4,350,955; 4,629,986; 4,717,877; 4,717,878; 4,717,876; 5,212,447; 5,280,243; and xe2x80x9cRemote xe2x80x98Inside Outxe2x80x99 NMRxe2x80x9d, J. Magn. Res., 41. p. 400, 1980; xe2x80x9cNovel NMR Apparatus for investigating an External Samplexe2x80x9d, Kleinberg el al., J. Magn. Res., 97, p. 466, 1992.
The apparatuses disclosed in the above documents are based on several permanent magnet configurations designed to create relatively homogeneous static magnetic fields in a region external to the apparatus itself. RF coils are typically used in such apparatuses to excite the nuclei in the homogeneous region and, in turn, receive the created NMR signal. To create an external region of homogeneous magnetic field, the magnetic configurations have to be carefully designed, to reconcile the fact that small deviations in structure may have a disastrous effect on magnetic field homogeneity. It turns out that such a region of homogeneous magnetic field can be created only within a narrow radial distance around a fixed position relative to the magnet configuration, and that the characteristic magnetic field intensities created in this region are generally low. As a result, such apparatuses, although permitting NMR measurements, have only limited use as imaging probes for imaging regions of the bore-hole walls.
With respect to medical MRI-based applications, the potential of using an intra-cavity receiver coil has been investigated, and is disclosed, for example, in the following publications: Kandarpa et al., J. Vasc. and Interventional Radiology, 4, pp. 419-427, 1993; and U.S. Pat. No. 5,699,801. Different designs for catheter-based receiver coils are proposed for insertion into body cavities, such as arteries during interventional procedures. These coils, when located in proximity of the region of interest, improve reception sensitivity, thus allowing high-resolution imaging of these regions. Notwithstanding the fact that this approach enables the resolution to be substantially improved, it still suffers from two major drawbacks: (1) the need for the bulky external setup in order to create the static homogeneous magnetic field and to transmit the RF excitation signal; and (2) the need to maintain the orientation of the coil axis within certain limits relative to the external magnetic field, in order to insure satisfactory image quality. Because of these two limitations, the concept of intra-cavity receiver coil is only half-way towards designing a fully autonomous intra-cavity imaging probe.
U.S. Pat. No. 5,572,132 discloses a concept of combining the static magnetic field source with the RF coil in a self contained intra-cavity medical imaging probe. Here, several permanent magnet configurations are proposed, for creating a homogeneous magnetic field region external to the imaging probe itself, a manner somewhat analogous to the concept upon which the bore-hole apparatuses are based. Also disclosed in this patent are several RF and gradient coil configurations that may be integrated in the imaging probe in order to allow autonomous imaging capabilities. The suggested configurations, nevertheless, suffer from the same problems discussed above with respect to the bore-hole apparatuses, namely, a fixed and narrow homogeneous region to which imaging is limited, and the low magnetic field values characteristic of homogeneous magnetic field configurations.
The main reason why the above-mentioned apparatuses, as well as most all NMR or MRI setups, prefer operation with a substantially homogeneous static magnetic field, is that such a setup allows operating in a narrow frequency bandwidth (typically a few Hz). Working in a narrow frequency band allows easier electronic circuit design for tuning and matching the receiver and transmitter channels, but, most importantly, it results in low noise or, rather, high signal to noise ratio (SNR) per NMR signal (spin-echo, for example). Since the field is substantially homogeneous, the duration of the NMR signal is typically a few milliseconds. The down side of this conventional scheme of working in homogeneous fields is that field homogeneity is hard to achieve over extensive volumes, especially for xe2x80x9cinside-outxe2x80x9d apparatuses.
The present invention takes an advantage of a technique described in a co-pending patent application assigned to the assignee of the present application. This technique is based on the operation in substantially non-homogeneous primary (static) magnetic field created by a probe device external to the probe in order to perform MRI of regions outside the probe, which are significantly larger than those obtainable with the prior art devices. The resulting wide bandwidth is more demanding in terms of electronic circuit and receiver/transmitter coil designs.
It is important to note that the lower SNR per spin-echo (due to large bandwidth) can be compensated for by acquiring a large number of such spin-echo signals (at least one hundred, and typically a few thousands) during one excitation series, the duration of which is roughly limited by a typical spin transverse relaxation time (known as T2). This stems from the fact that each wideband echo is very short in time, when compared with a narrowband echo. This realization, together with special hardware design, enables NMR imaging and measurements over extensive regions of non-homogeneous static magnetic fields.
The present invention is aimed at designing a novel probe device and an imaging method utilizing the same, characterized by operation with substantially non-homogeneous primary magnetic field, for use in MRI systems. The probe of the present invention is a fully autonomous intra-cavity MRI probe, comprising all components necessary to allow magnetic resonance measurements and imaging of local surroundings of the probe, obviating the need for external magnetic and electromagnetic field sources.
One of the suitable applications of the invented probe is the imaging of blood vessel walls. The imaging probe in such application is preferably integrated in an intravascular catheter, which is connected to an external imaging console.
The imaging method according to the invention is based on the creation of substantially non-homogeneous primary magnetic field (e.g., by permanent magnets), and the use of wideband reception and transmission channels, as well as specifically designed excitation pulse sequences. The present invention enables to achieve high SNR (similar to that of the conventional technique) by accumulating a large number of wide-bandwidth short duration echoes, in a similar overall acquisition time.
For the purpose of better understanding the imaging method according to the present invention, the following should be defined:
The term xe2x80x9cregion of interestxe2x80x9d (ROI) signifies a region of the medium to be imaged. For intravascular applications, ROI is typically a segment of the blood vessel wall, extending 360 deg in circumference, a certain xe2x80x9cpenetrationxe2x80x9d distance radially into the vessel wall, and a certain length along the vessel.
The ROI may be formed by a plurality of locally adjacent cross-sectional slices, the so-called xe2x80x9cimaging slicesxe2x80x9d, having different positions along the length of the blood vessel (i.e., longitudinal axis of the probe).
Each slice may be further divided into angular imaging sectors. Each sector may be imaged, as will be explained in detail, by a sequential excitation and signal reception from a set of sub-regions thereof, each set covering an imaging sector.
There is thus provided according to one aspect of the present invention, an imaging probe for use in an MRI system, the probe comprising:
(a) a first magnetic field source for creating a primary, substantially non-homogeneous, static magnetic field in a region of a medium outside the probe;
(b) a second magnetic field source for creating an external time-varying magnetic field, the second magnetic field when being applied to said region, is capable of exciting nuclei in an extended sub-region of the medium to generate NMR signals;
(c) a receiver for receiving the NMR signals and generating data indicative thereof.
In order to image the ROI, the probe is placed in the medium such that the static magnetic field region created by the first magnetic field source outside the probe is located within the region of interest.
Preferably, the first magnetic field source comprises a pair of permanent magnet assemblies spaced-apart from each other along the probe axis (Z-axis), wherein the assemblies are magnetized in the opposite directions along an axis perpendicular to the probe axis (X-axis). The primary magnetic field is preferably created in only a sector of the imaging slice, defined as the imaging sector, and in this sector the field is substantially parallel to the Z-axis, and its strength decreases monotonously and substantially as one moves along the X-axis away from the probe edge, and therefore can be characterized by a substantial field gradient along the X-axis. For example, this gradient can be about 30 T/m within an imaging sector of about 2 mm""s in the radial dimension.
It should be understood that the non-homogeneity of the primary magnetic field along the X-axis will result in short NMR signals (typically spin-echoe signals), namely, significantly shorter than those obtained with the prior art devices operating within the region of substantially homogeneous primary field. For example, a conventional spin echo may have duration of a few tens of milliseconds, while the spin echoes created by said probe may be only a few microseconds wide. This allows the acquisition of a few thousands of spin-echo signals in one excitation series having duration of a typical transverse relaxation time (T2). Accordingly, the averaging of such a great number of signals enables to increase significantly a signal-to-noise ratio in the final signal to be analyzed. The SNR compensation achieved by averaging, therefore, enables the operation with the non-homogeneous primary field, which can correspond to a frequency bandwidth of more than 1% of the mean value of a magnetic resonance frequency of nuclei, but which can go as high as almost 200% of mean frequency value. For example, the above X-gradient primary field (30 T/m) corresponds to a magnetic resonance frequency range of 3.5 MHz to 6 MHz, i.e., a bandwidth to center frequency ratio of about 50%.
The time-varying magnetic field created by the second magnetic field source (which is formed by at least one RF coil) his sufficiently high intensity and sufficiently wide frequency band, such that it is capable of efficiently and simultaneously exciting nuclear spins to generate NMR signals characterized by NMR frequency bandwidth of more than 1% (generally, 5%-200%, typically about 10%) of their mean frequency value. Since, as stated above, the magnetic resonance frequency range of 3.5 MHz to 6 MHz (a difference of 2.5 MHz) corresponds to a bandwidth to center frequency ratio of about 50% then a 5% frequency bandwidth corresponds to 250 kHz. It should be understood that the term xe2x80x9csimultaneous excitationxe2x80x9d signifies excitation by a single pulse. Accordingly, the receiver has also sufficiently high sensitivity and sufficiently wide frequency band, similar to that of the transmitting coil.
Preferably, the at least one RF transmitting coil is a flat solenoid mounted on the probe edge such that the axis of the solenoid coincides with the X-axis.
For obtaining two-dimensional images of an imaging slice in the region of interest, a third magnetic field source may be used for creating the so-called xe2x80x9cxcfx86-gradientxe2x80x9d, which corresponds to an angular coordinate varying magnetic field. Preferably, this third magnetic field source includes a pair of parallel RF solenoidal coils, which are arranged such that their axis are parallel to the Z-axis, and located in the probe itself and close to the probe edge.
The xcfx86-gradient may be created by a magnetic field of approximately linear dependence on the angle in the imaging sector, and can be activated in the imaging sequence. Gradient coil activation can be done in several ways. One alternative is to use double polarity gradient pulses before and after each spin echo, i.e., a positive gradient pulse before echo acquisition, and a similar but opposite (negative) pulse after the echo. In this case, the echo signal is phase-encoded in the angular dimension, and then corrected to the original phase following each echo. By varying the strength or duration of each pair of gradient pulses, while a static radial gradient (along X-axis) exists, a complete scan of K-space can be obtained.
When the imaging of a sector is completed, the probe can be rotated around its axis (Z-axis) in order to image other sectors in the same slice, thereby covering the entire imaging slice. Data obtained from the entire acquisition procedure can be processed in real-time and a two-dimensional image can be reconstructed and displayed on a PC monitor.
In order to create a three-dimensional image of the region of interest, the probe can be moved along the Z-axis or along a channel (if for example the latter is a curve-like). Alternatively, multiple probes can be aligned along the Z-axis, or along a curved-like channel.
According to another aspect of the present invention, there is provided an imaging probe for use in an MRI system, the probe comprising:
a first magnetic field source for creating a primary, substantially non-homogeneous, static magnetic field in a region of a medium outside the probe;
a second magnetic field source for creating an external time-varying magnetic field having intensity and frequency range such as to, when being applied to said region, efficiently and simultaneously excite nuclear spins to generate NMR signals characterized by NMR frequency bandwidth of more than 1% of their mean frequency value; and
a receiver for receiving the NMR signals and generating data indicative thereof;
the probe thereby enabling acquisition of up to a few thousands of spin-echo signals in one excitation series that has a duration of a typical transverse relaxation time (T2).
According to yet another aspect of the present invention, there is provided an imaging system comprising an intravascular magnetic resonance imaging catheter and a complementary external console associated with electronics for generating transmitted signals and processing received signals, the catheter comprising a fully autonomous imaging probe mounted at the distal end of the catheter for imaging a region of interest surrounding the distal end of the catheter, wherein the probe is characterized by the above features (a) to (c).
The electronics mainly includes the following elements: an electronic module for tuning the receiver channel and matching it to the receiver coil, and for pre-amplifying the NMR signal received by said receiver coil, a module for matching transmission channel to the second magnetic field source, a high-power amplifier for amplifying the transmission pulses to the required high voltage/current, a gradient generator unit for generating gradient field pulses at the required levels, and an external unit for further amplifying the pre-amplified received NMR signal.
The system also comprises a connector unit for connecting the intravascular catheter to the external console, both mechanically and electrically. A motor unit is provided for rotating the imaging probe about its axis in order to create two-dimensional images, and preferably also for moving the probe along its axis in order to obtain three-dimensional images of segments of the ROI (i.e., successive imaging slices) in the blood vessel wall.
The external imaging console comprises preferably a personal computer (PC) with installed hardware for producing analog transmission signals, for recording digitally the NMR received signals, for processing the signals into images, and for displaying the obtained images on the PC monitor.
When the catheter connected to the external imaging console is inserted into the blood vessel, the position of the probe is monitored by available equipment in the catheterization or operating room, such as X-Ray angiography. When the imaging probe is located adjacent to a vessel wall segment, which needs to be imaged, the operator initiates the imaging process via the imaging console controls.
The acquisition of images of the region of interest is enabled by the following: The first magnetic field source creates a primary static non-homogeneous magnetic field having a sufficient strength in the region of interest (around {fraction (1/10)} Tesla) outside the probe. This magnetic field region can be visualized as a sector of a plane perpendicular to the catheter axis (imaging slice). The second magnetic field source is used to excite the nuclei located in a sub-region of said sector of the imaging slice, while the receiver is used to receive the NMR signal (typically an echo-signal) produced by the nuclei.
In order to produce a two-dimensional image of the imaging slice-, two magnetic field gradients are used: The first gradient is generally radial in direction, and is inherent in the static magnetic field profile, which generally drops substantially in strength when moving away from the probe edge. The second gradient is time-dependent and is generally directed perpendicularly to the radial gradient, i.e. the angle direction. The time varying gradient is preferably produced by a pair of parallel solenoidal coils, located in the probe itself and close to the probe edge.
A sequence of excitation pulses, preferably a multiple spin echo sequence, such as the well known CPMG, is transmitted by the second magnetic field source. This sequence has the advantage that a large number of spin-echoes can be recorded during a time frame of signal coherence, even at substantial gradients of the static and RF fields. Excitation of nuclei within the sector-shaped region is preferably done by dividing this region into sub-regions, such that each sub-region includes nuclei corresponding to a narrower resonance frequency band, as compared to that of the entire sector-shaped region. Different sub-regions of the sector-shaped region are generally located at different radial distances from the probe edge. Therefore, in order to excite a specific sub-region, the carrier frequency of each transmission pulse burst generated by the second magnetic field source is determined according to the specific resonance frequency of the nuclei residing in that sub-region. By stepping the carrier frequency from one pulse burst to the next, the entire sector-shaped region at the current location of the probe can be scanned. Using extremely short pulses of sufficient power allows each sub-region to be substantially wide, meaning that a single pulse can excite nuclei in a substantially wide radial distance. Therefore, an image of the entire sector-shaped region can be acquired by a few pulse bursts, and by rotating the probe the entire imaging slice can be scanned and imaged.
The NMR signal received in each spin echo can be readily transformed into a one-dimensional density profile (neglecting NMR relaxation and diffusion effects) of the nuclei in the excited sub-region, along a radial vector. This is because each echo is acquired under a static (inherent) radial field gradient.
For obtaining two-dimensional images, the angular gradient coils, which create a magnetic field of approximately linear dependence on the angle in the imaging sector, can be activated in the imaging sequence. Gradient coil activation can be done in several ways, as will be obvious to the artisan. One option is to use double polarity gradient pulses before and after each spin echo, i.e. a positive gradient pulse before echo acquisition, and a similar but opposite (negative) pulse after the echo. This way the echo signal is phase-encoded in the angular dimension, and then corrected to the original phase following each echo. By varying the strength or duration of each pair of gradient pulses, while a static radial (read-) gradient exists, a complete scan of K-space can be obtained. When imaging of a sector-shaped region is completed, the probe can be rotated around its (and the catheter""s) axis in order to image other sectors, therefore covering the entire imaging slice. The data obtained from the entire acquisition procedure can be processed in real-time and two-dimensional images can be reconstructed and displayed on the PC monitor.
In order to create a three-dimensional image, the probe can be moved along its axis, or alternatively, multiple probes can be located along this axis, covering simultaneously a long vessel segment.
The probe is further connected to the electronic module, which is preferably located in proximity to the probe itself. The electronics module is used to amplify the received signal up to a point where further noise or interference accumulated along the way becomes negligible. The electronics module also comprises basic tuning and matching components of the receiver channel, along with protection components at receiving pre-amplifier inputs. Additional wires connect the probe to following modules of both receiver and transmission channel, which do not have to be located in proximity to the probe. For intravascular applications, it is possible to locate only the probe, pre-amplifying (electronics) and matching capacitor modules in the catheter (which is usually limited in diameter). All other system modules can be located externally to the body and, therefore, with fewer constraints.
The external modules operate in the following way: The pulse sequence is generated in the PC using digital to analog hardware. The analog transmission pulses are amplified by the high power amplifier and driven into the transmission coil inside the catheter via the interface unit and transmission tuning capacitor. The NMR signals received by the receiver coil are pre-amplified by the pre-amplifying unit in the catheter and then by additional amplification stages outside the catheter. The analog received signal is digitized in the PC and acquired into memory. It is then processed in order to give a full two- (or three-) dimensional image, which is displayed on the PC monitor. The pulse sequence generated by the PC also controls gradient field pulse generation using the gradient generator module.
According to yet another aspect of the present invention, there is provided an imaging method for creating an image of a region of interest, the method comprising the steps of:
(i) creating a primary, substantially non-homogeneous, static magnetic field in the region of interest;
(ii) creating a time-varying magnetic field within an extended sub-region of the region of substantially non-homogeneous static magnetic field, wherein said time-varying magnetic field has intensity and frequency band such as to simultaneously and effectively excite nuclei in the sub-region to generate NMR signals characterized by NMR frequency bandwidth of more than 1% of their mean NMR frequency value;
(iii) receiving the signals and generating data indicative thereof.
The main conceptual novelty of this invention is that the imaging probe utilizes a substantially non-homogeneous magnetic field, which may be produced either externally or internally to the field sources, as its static magnetic imaging field. In addition, the imaged region (sector) of the region of interest (ROI) is substantially extensive, relative to the dimensions of the field sources in general, or, when the sources are contained in an imaging probe, relative to the dimensions of the imaging probe. The combination of these two requirements means that both hardware characteristics and imaging method, which are part of the disclosed invention are very unique and non-conventional in the context of ordinary MRI. These characteristics include inter alia the following:
A design of both transmission and receiver channels, so that they can operate in a simultaneous wide band of frequencies.
A design of an extremely sensitive receiver channel, including an extremely low noise pre-amplifier, matched to a sensitivity-optimized coil over a wide frequency band. The requirement for high sensitivity stems from the requirement to overcome the high noise levels characterizing the wide frequency band.
A design of imaging sequences utilizing the short echo duration, againxe2x80x94following the wide frequency band. The short echo duration allows averaging of thousands of echoes in a single coherence duration (T2) of a few tens of milliseconds. This allows averaging and, therefore, a significant improvement in the final SNR.
It is important to note that according to the invention, the static magnetic field source itself serves as a means for creating a magnetic field gradient to image the region of interest along one dimension thereof. A gradient creation means are typically utilized in an MRI system, and always as a separate element, in addition to static and time-varying RF magnetic field sources.
Thus, according to yet another aspect of the present invention, there is provided an imaging probe for use in an MRI system, in which an image of a region of interest in a medium along it least one dimension of said region is created by utilizing a means for producing at least a first magnetic field gradient along said at least one dimension, the imaging probe comprising:
a first magnetic field source for creating a primary static magnetic field in the region of interest;
a second magnetic field source for creating an external time-varying magnetic field to exciting nuclei in an extended sub-region of the medium to generate NMR signals; and
a receiver for receiving the NMR signals and generating data indicative thereof;
wherein said primary static magnetic field created by the first magnetic field source is substantially non-homogeneous, said first magnetic field source thereby operating as the means for creating the first magnetic field gradient.